Currently, proper broadband access lines have been provided based on XDSL or FTTH technologies and access lines having a high transmission capability are spreading for household use.
Therefore, high quality access lines can now be commonly used at home. However, there is no established technology for home network, which is at one end of the access lines.
General users can construct a fast and high quality network in a household by installing a high frequency dedicated communication line such as Ethernet (registered trademark) or the like. However, it is difficult and costly for general users to newly install an Ethernet cable in an existing house.
Obviously, it will be highly advantageous for these users to construct a fast digital network at low cost without installing a physical cable, if possible.
On the other hand, home network appliances that general users can easily use without any specific expertise have been actively developed. It is now urged to provide proper digital network infrastructure in households using such home network appliances.
As a solution enabling the general users to construct a fast digital network at low cost without newly installing a dedicated communication cable physically, a method of using wired cables such as existing electric light wires, telephone lines, TV antenna lines or the like and a method of using wireless networks have been proposed.
However, these communication media are not intended for fast digital communication, and so have disadvantages that, for example, sufficient bands are not guaranteed or communication lines are not reliable.
Uses of the network in households include stream distribution which needs to be fast and does not permit data loss, such as high definition (HD transmission) quality AV video; digital phone or the like which requires strict restriction against delay, and web browsing or the like for which quality guarantee restriction is loose.
Accordingly, it is expected that for the network used in households, various levels of capability or quality are desired.
In addition, considering that networks are used in the households in order to receive the video distribution service using the IP protocol such as IP-TV system which has recently become common, the IP multicast packets need to be transferred stably and at high speed.
The following technique is one technique to realize a fast and high quality transmission system using a medium which is not intended for high frequency digital communication.
First, a terminal divides the frequency band used for the communication to define sub carriers, and issues a training packet to evaluate the communication quality of the transmission path corresponding to the frequency band of each sub carrier. Next, the terminal determines the number of bits per predetermined time which are to be described for each sub carrier. Information on the number of bits to be described for each sub carrier is referred to as a “transmission map”.
Here, a form by which one terminal and another terminal are connected one to one is referred to as a “1:1 link”. A form by which one terminal is connected to n number of terminals is referred to as a “1:n link”.
The terminal determines the communication speed of the 1:1 link based on the number of bits described in the transmission map using a sub carrier in a frequency band having a high communication quality of the transmission path. By this, the signal-to-noise property can be improved to perform an optimum fast transfer.
However, the technique of increasing the speed by optimizing the modulation/demodulation format in the 1:1 link is not suitable to the transfer in the n:1 link. The reason is that because different transmission paths have different characteristics, a transmission map optimum for the transfer in one 1:1 link is not necessarily optimum for the transfer in another 1:1 link.
As a technology for increasing the speed of the inter-link transfer in the 1:n link in response to an IP multicast or IP broadcast transfer request, the following technology has been published.
A 1:n link includes a plurality of 1:1 links. First, the number of bits which can be described in each of the sub carriers in each 1:1 link included in the 1:n link is measured, and the transfer map of each link is estimated.
Then, a logical product is calculated for the sub carrier of each transmission map, and the sub carriers which can be used commonly among the links are determined. A common number of bits which can be assigned to each of the sub carriers is determined, and the determined common number of bits is used for the multiple access communication in the 1:n link (for example, Patent Document 1).
Patent document 1: Japanese Laid-Open Patent Publication No. 2001-111518